Preparations for tinting and coloring hair are an important type of cosmetic product. They can be used to lighten or darken the natural shade of hair color according to the wishes of the particular user, to obtain a completely different color, or to cover unwanted color tones, for example grays. Depending on the color or the duration of the coloration desired, conventional hair colorants are formulated either on the basis of oxidation dyes or on the basis of substantive dyes. In many cases, combinations of oxidation dyes and substantive dyes are also used to obtain special shades.
Colorants based on oxidation dyes lead to brilliant and long-lasting colors. However, they do involve the use of strong oxidizing agents, for example hydrogen peroxide solutions. Dyes of this kind contain oxidation dye precursors, so-called developer components as well as coupler components. Under the influence of oxidation agents or atmospheric oxygen, the developer components form the actual dyes together with one another or by coupling with one or more coupling components.
Colorants based on substantive dyes are frequently used for temporary colors. Substantive dyes are based on dye molecules which are directly absorbed onto the hair and do not require an oxidative process for developing the color. Important representatives of this class of dyes include triphenylmethane dyes, azo dyes, anthraquinone dyes or nitrobenzene dyes, each of which can bear both cationic or anionic groups.
In the course of the colorization processes, it can for a variety of reasons be the case that the colorization must be partially or completely reversed. For example, a partial removal of the colorization becomes appropriate if the resulting color of the fibers is darker than desired.
On the other hand, a complete removal of the coloring may be desirable in some cases. For example, it is conceivable that hair will be dyed or tinted for a certain occasion, but is intended to return to its original color after several days.
Methods for color removal are already known in the literature. One method well-known from the prior of art for the reversal of colorizations is an oxidative process for after-treating the dyed hair, for example with the aid of a conventional bleaching agent. During this process, however, the fibers can become damaged due to the introduction of strong oxidizing agents.
Furthermore, reductive processes for the removal of color have already been described. For example, European Patent Application EP 1 300 136 A2 discloses a method for hair treatment in which the hair is colored in a first step and the color reductively removed in a second step. The reductive decolorization thereby is effected by applying a formulation containing a dithionic salt and a surfactant. In WO 2008/055756 A2, the reductive decolorization is performed using a mixture of a reducing agent and an absorbing agent.
The decolorization occurs by using reductive decolorization agents to reduce the dyes found on the keratin fibers or hair. Due to the reduction, the dyes are normally transformed into their reduced leuco forms. During this process, the double bonds present in the dyes are reduced in such a way that the chromophore system of the dyes is interrupted, and the dye is converted to a colorless form.
An overall problem with the reductive decolorization agents known from the prior art is that, although the color can initially be removed from the dyed keratin fibers through the use of the reducing agent, the color removal is not permanent. In some cases, realistic and high-quality colorizations are achieved with hair that has been dyed in an oxidative process whereby the coloration is generated in the hair using oxidation dye precursors of the developer type and the coupler type. However, by applying the reductive decolorization agent, these dyes are then reductively transformed into colorless compounds, which, by virtue of their authenticity and high-quality, continue to remain on the hair.
After the reducing agent has been rinsed away, these reduced forms can gradually be reoxidized under the influence of atmospheric oxygen. As a result of this reoxidation, a more or less pronounced recoloration takes place. This recoloration does not usually correspond to the shade with which the keratin fibers had previously been dyed, and may turn out to be unexpectedly unattractive, thus less than desirable to the user of the decolorization agent.